1. Field of the Invention
The present invention relates to transmission of high laser power densities through modification of optical fiber end surfaces, thereby increasing the damage threshold of the system and increasing the power density available at a work or treatment site. In particular, the modifications involve micro-structuring the optical fiber end surfaces.
2. Information Disclosure Statement
High power laser systems of up to 6 kW are currently available for several manufacturing tasks, such as laser cutting, welding, and marking and even higher power systems are under development. High laser beam power density is a critical requirement for these tasks and the transmission of this laser energy over long (up to several hundred meters) distances is required to share costly laser sources between work stations on a manufacturing line.
Since laser power density is inversely proportional to the beam cross section, optical fibers with small core cross sections are needed to transmit the highest power density available to a work or treatment site although the maximum power density is limited by the power source. The launch of high laser power density into increasingly smaller optical fiber cross sections creates problems such as destruction of the optical fiber end surfaces. This destruction results from several phenomena. First, the absorption at the end surface of even a very pure optical fiber material is substantially higher than the absorption throughout its volume. This surface absorption is intensified during the manufacturing tasks described above because materials surrounding the output end surface tend to absorb laser light at that frequency.
Second, due to the physics of light reflection between materials having different refractive indices, the power density near the input and output surfaces becomes concentrated. That is, an incident laser beam wave (having an amplitude A.sub.i) that leaves a first media (with a refractive index n.sub.1) and perpendicularly enters a more dense media (with a refractive index n) will generate interference in the form of a reflected wave with an amplitude A.sub.r. This results in a refracted or transmitted wave having an amplitude A.sub.o (A.sub.o =(A.sub.i -A.sub.r)). The power density at the end surface is proportional to the square of A.sub.o (A.sub.o.sup.2 =(A.sub.i -A.sub.r).sup.2). For a wave traveling from the more dense media, the amplitudes of the incident and reflected waves are compounded, and thus, the intensity of the laser radiation in the output end surface layer is proportional to A.sub.o.sup.2 =(A.sub.i +A.sub.r).sup.2. The difference in power density near the input and output end surfaces quickly grows according to the refractive index ratio n/n.sub.1 because of the light reflected from the end surfaces quantified by intensity R where R=(n/n.sub.1).sup.2.apprxeq.((n-n.sub.1)/(n+n.sub.1)).sup.2. Thus reflective losses greatly contribute to laser induced damage.
Due to the above problems, damage thresholds of optical fiber end surfaces are considerably lower than the theoretical internal damage threshold which is defined as equivalent to the bond strength of the optical material. For example, the theoretical damage threshold for SiO.sub.2 is 10-50 GW/cm.sup.2 which correlates to the Si--O bond strength.
Means to increase the damage threshold of optical fiber end surfaces have previously been suggested. Goldberg et al. proposes decreasing the apparent density of energy at the optical fiber end surface by using a funnel shaped input end or a water filled cavity at the input end (U.S. Pat. No. 4,641,912). The increased area of the funnel shaped energy coupler decreases the input power density for a given level of power within the optical fiber. By decreasing the cross sectional area of the optical fiber after the energy has been coupled in, the density of the power can then be increased. However, although the apparent density of energy at the input surface is somewhat reduced, the Fresnel reflection losses remain constant.
Fabrication of a spherical micro-lens directly on the optical fiber by manufacturing a taper on the fiber end, cutting the taper at a specific distance, and melting its end to provide a spherical shape, can also increase transmittance (U.S. Pat. No. 5,011,254). This prior art is illustrated in FIG. 1. High laser power density radiation 10 is launched at thermally deformed input end 14 and radiation is guided along core 12 whose diameter is smaller than deformed input end 14. This arrangement simplifies the system and minimizes the number of optical interfaces between laser and optical fiber, but the method is very operator dependent, difficult to reproduce, and does not provide a high quality spherical surface on the fiber end surface.
Other known methods of shaping optical fiber end faces are based on polishing techniques. FIG. 2 presents a further example of prior art where a thermally deformed input end 24 is polished. High laser power density radiation 20 is launched towards thermally deformed input end 24 that has been polished flat to end surface 26. Radiation is guided along core 22 whose diameter is smaller than thermally deformed input end 24. The polishing procedure is difficult to control and may fail to provide an optimal shape on the optical fiber end surface which can lead to increased reflection losses. Ideally, a surface on the optical fiber end surface should have a hyperbolic shape in the direction corresponding to that of large beam divergence while a typical polishing technique provides a primarily spherical optical surface (US. Pat. No. 5,751,871).
U.S. Pat. No. 5,602,947 issued to Wolfgang Neuberger and hereinafter incorporated by reference discloses a method for providing micro-voids that make a optical fiber end surface substantially non-reflective, thereby decreasing Fresnel reflection losses for a predetermined wavelength. However, the optical fiber material must have similar mechanical and thermal properties to those of the mid-IR fibers. Additionally, in the mid-IR region, micro-structures can be significantly larger and easier to manufacture than micro-structures in the 1 .mu.m range where high power solid state lasers and quartz glass fibers typically operate.
Reflection losses can also be decreased by depositing a anti-reflective coating on the surface of the highly reflective material. However, in practice, such a technological operation is complicated, time-consuming and expensive. Additionally, most anti-reflective coating films are very delicate and sensitive to possible contamination during the depositing process. There is thus a need for a laser transmission system that improves on the state of the art by allowing increased high laser power density transmission, and thereby providing increased power density available for manufacturing tasks such as laser cutting, welding, and marking.